Messages in this thread | | | Subject | Re: [RFC] New timeofday proposal (v.A1) | From | john stultz <> | Date | Wed, 08 Dec 2004 16:17:23 -0800 |
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On Wed, 2004-12-08 at 15:53, Christoph Lameter wrote: > On Wed, 8 Dec 2004, john stultz wrote: > > However, there is also this short term single shot adjustments. These > > adjustments are made by applying the MAX_SINGLESHOT_ADJ (500ppm) scaling > > for an amount of time (offset_len) which would compensate for the > > offset. This style is difficult because we cannot precompute it and > > apply it to an entire tick. Instead it needs to be applied for just a > > specific amount of time which may be only a fraction of a tick. When we > > start talking about systems with irregular tick frequency (via > > virtualization, or tickless systems) it becomes even more problematic. > > We would need to schedule a special tick like event at a certain time but > otherwise I do not see a problem. Is there a requirement that these > "specific amounts of time" are less than 1 ms? The timer hardware (such as > the RTC clock) can generate an event in <200ns that could be used to > change the scaling. For a tickless system we would need to have such > scheduled events anyways.
Eh, I'd like to not to be dependent on event accuracy/frequency. Lets see if we can do it w/o scheduling events.
> > If this can be fudged then it becomes less of an issue. Or at worse, we > > have to do two mult/shift operations on two "parts" of the time interval > > using different adjustments. > > That looks troublesome. Better avoid that.
Well, its not *that* bad. Similar to the ntp_scale() function, it would look something like:
if (interval <= offset_len) return (interval * singleshot_mult)>>shift; else { cycle_t v1,v2; v1 = (offset_len * singleshot_mult)>>shift; v2 = (interval-offset_len)*adjusted_mult)>>shift; return v1+v2; }
Where: singleshot_mult = original_mult + ntp_adj + ss_mult and adjusted_mult = original_mult + ntp_adj
> > Its starting to look doable, but its not necessarily the simplest thing > > (for me at least). I'll put it on my list, but patches would be more > > then welcome. > > I am still suffering from my limited NTP knowlege but will see what I can > do about this.
:) Any added NTP knowledge would be great to add to the pool.
-john
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